It has been known for some time that, when people reminisce in a group, an error in memory held by one group member may be transferred to others in the group.

Sometimes, people genuinely believe the new erroneous version of events, a process known as 'rewriting'. In other circumstances, people will outwardly agree with the incorrect story, but inwardly hang on to their original true memory. They are 'pretending' they share the memory.

"Humans are very social animals," says Edelson. "We are exposed to social influences constantly; whether it is talking to friends or going to the grocery shop. So, it is important that we understand how our brains are wired to deal with socially-affected memories."

Edelson divided 30 volunteers into small groups to watch a TV documentary. Three days later, he individually tested their memories about the program with a 400-item questionnaire.

Four days after that, the subjects were tested again, while lying in a functional magnetic resonance imager (fMRI). This time, they were shown fabricated information for some questions, suggesting that all the other members of their group had come up with an answer contradicting their own.

The subjects changed their answers to fit with the majority view on almost 70 per cent of occasions.

Changes in the brain

Further tests suggested that for most of the altered questions, people still had their original memory intact and were just pretending or 'fitting in'. However, in about 40 per cent of cases, the memory seemed to have been rewritten to adopt the majority view.

The two different routes, rewriting and pretending, showed up quite differently on the brain scans. When the participants 'rewrote' their memories to incorporate the false information, there was heightened activity in the hippocampus and the amygdala. When they were pretending, greater activity was recorded in part of the cingulate cortex.

The hippocampus is known to be important in memory, but heightened activity in the brain's emotional centre, the amygdala, was unexpected.

Edelson thinks the work could have implications for eyewitness testimony in law.

"A British study suggested that [the majority] of eye-witnesses to a crime are not alone, and that many eye-witnesses talk to each other. Our study suggests that this may sometimes influence their memories of the event."

Not always a bad thing

Commenting on the study, Professor Henry Roediger and Associate Professor Kathleen McDermott of Washington University in St Louis say the results are, in some ways, surprising.

"The observation that amygdala activation occurred disproportionately on occasions when social influence consistently changed memory ... suggests a previously unrecognised function for this brain region," they write.

Roediger and McDermott also believe that "memory conformity may typically be beneficial".

"If one individual in a group forgets critical information (about food resources or dangers, for instance), then it is wise to get an updated memory from another group member."